Known in the art is a spark ignition type internal combustion engine provided with a system able to change a combustion chamber volume wherein when a temperature of a catalyst arranged inside an engine exhaust passage falls and activity will be lost, the combustion chamber volume is increased to lower the actual compression ratio and thereby cause the combustion efficiency to deteriorate and the exhaust gas temperature to rise and thus raise the temperature of the catalyst (for example, see Japanese Patent Publication (B) No. 4-28893).
However, if lowering the actual compression ratio to raise the temperature of the catalyst, there is the problem that the ignition and combustion of the fuel deteriorate, so stable combustion can no longer be obtained.